Text Description:St. George Island, Group 1 type rocks (from Chang and others, 2009): "Group 1 rocks are commonly crystal poor (5-15 vol. % phenocrysts) and vesicular, with fine-grained, holocrystalline groundmasses. Phenocrysts are primarily olivine, although some rocks have clinopyroxene or opaque phenocrysts as well. The olivine phenocrysts are commonly equant or skeletal and vary widely in size and form from sample to sample and within rocks. Olivine compositions for seven rocks range from Fo90 to Fo61 and average Fo83. Clinopyroxene phenocrysts are generally small and euhedral and have average core and rim compositions of XMg=70 and 62, respectively. Clinopyroxene phenocrysts vary in color from tan to pink in plane-polarized light and often exhibit sector-zoning and occur in X- or star-shaped glomerocrystic clusters. The groundmasses generally consist of olivine, euhedral clinopyroxene, euhedral plagioclase laths, and opaque minerals. Modal nepheline was found in one rock (SG06-8). Abundant groundmass clinopyroxene is especially notable in group 1 rocks. The average clinopyroxene in the groundmass has a composition of XMg=66. Plagioclase compositions measured in two group 1 rocks are variable. The average plagioclase composition in sample SG05-28 is Ab36Or3An61, with two analyzed crystals having anomalous rims with an average composition of Ab48Or37An15. Plagioclase analyzed in sample SG05-31 is albite (Ab84Or12An3)."

St. George Island, Group 1 type rocks (from Chang and others, 2009): "Group 1 rocks are commonly crystal poor (5-15 vol. % phenocrysts) and vesicular, with fine-grained, holocrystalline groundmasses. Phenocrysts are primarily olivine, although some rocks have clinopyroxene or opaque phenocrysts as well. The olivine phenocrysts are commonly equant or skeletal and vary widely in size and form from sample to sample and within rocks. Olivine compositions for seven rocks range from Fo90 to Fo61 and average Fo83. Clinopyroxene phenocrysts are generally small and euhedral and have average core and rim compositions of XMg=70 and 62, respectively. Clinopyroxene phenocrysts vary in color from tan to pink in plane-polarized light and often exhibit sector-zoning and occur in X- or star-shaped glomerocrystic clusters. The groundmasses generally consist of olivine, euhedral clinopyroxene, euhedral plagioclase laths, and opaque minerals. Modal nepheline was found in one rock (SG06-8). Abundant groundmass clinopyroxene is especially notable in group 1 rocks. The average clinopyroxene in the groundmass has a composition of XMg=66. Plagioclase compositions measured in two group 1 rocks are variable. The average plagioclase composition in sample SG05-28 is Ab36Or3An61, with two analyzed crystals having anomalous rims with an average composition of Ab48Or37An15. Plagioclase analyzed in sample SG05-31 is albite (Ab84Or12An3)."